1. Field of the Invention
This invention relates to an etching solution for a tin-nickel alloy. More particularly, it is concerned with an etching solution capable of selectively etching a tin-nickel alloy when the tin-nickel alloy coexists with copper. The invention also relates to a process for etching a tin-nickel alloy.
2. Description of the Prior Art
A printed circuit board with resistors, which is produced using a laminated material wherein a resistor film is provided on at least one side of an insulative support and a high conductive layer such as copper is provided on the resistor film, is well known in the art.
Recently, as a resistor film of a printed circuit board, a nickel film, a nickel-phosphorus film, a nickel-molybdenum film, etc., which are formed by electroplating, have been proposed and employed. These films, however, generally have many problems in terms of their physical properties, workability and so on.
The inventors have developed a printed circuit board with resistors in which a tin-nickel alloy film formed by electroplating is used as a resistor film. This printed circuit board is disclosed and explained in detail in Japanese Patent Application (OPI) No. 72468/79 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application").
In general, the plating of a tin-nickel alloy is carried out by chemical plating wherein a hydrazine hydrate, sodium hypophosphite or the like is used as a reducing agent; electroplating using a fluoride bath wherein sodium fluoride, acidic ammonium fluoride or the like, or a pyrophosphoric acid bath containing tin pyrophosphate, potassium pyrophosphate or the like, and so on. In plating the tin-nickel alloy for forming the resistor film of the printed circuit board, electroplating is preferably employed since the film thickness and the composition of bath are more easily controlled. In particular, electroplating using the fluoride bath is preferred in that over a wide range of bath compositions, an alloy containing tin and nickel in substantially equiatomic ratio (i.e., 64 to 70% by weight tin) is obtainable.
The tin-nickel alloy film is excellent in socalled smoothness and uniform electroplating as well as in the stability of its composition and, therefore, it is an excellent resistor film. However, many difficulties are encountered in producing the printed circuit board with resistors by selectively etching the tin-nickel alloy film on a copper film because it exhibits a quite high resistance against etching.
The printed circuit board with resistors in which the tin-nickel alloy is used as the resistor film is produced by, for example, the following procedure as described in Japanese Patent Application (OPI) No. 72468/79.
First, a tin-nickel alloy is plated on one side of a copper foil and on the thus plated tin-nickel alloy film, an insulative support, such as a glass cloth impregnated with an epoxy resin, is placed and bonded to produce a laminated plate material. The surface of the copper foil in the laminated plate material is covered with a photoresist. The photoresist is then exposed to light through a photomask having a combined pattern of conductor and resistor areas and developed. Thus, the photoresist is left corresponding to the pattern, and at the areas not covered with the photoresist, the copper foil is removed by etching, exposing the surface of the tin-nickel alloy. The thus exposed tin-nickel alloy film is also removed by etching, exposing the surface of the insulative support.
Then, the remaining photoresist is removed, and the surface of the laminated plate material is again covered with a fresh photoresist. In the same manner as described above, a photomask having a conductor pattern is employed and the photoresist is left corresponding to the conductor pattern of the photomask. At the areas not covered with the photoresist, the copper foil is removed, exposing the tin-nickel alloy film. Thereafter, on removing the remaining photoresist, a circuit corresponding to the above conductor and resistor pattern is formed on the insulative support, and thus a printed circuit board with resistors is obtained.
Therefore, in producing the printed circuit board with resistors wherein the tin-nickel alloy film is used as the resistor film, it is evident that the copper should not be etched at the time of etching the tin-nickel alloy film. That is to say, an etching solution for use in etching the tin-nickel alloy is required to have no capability of etching copper. When the etching solution for the tin-nickel alloy is able to etch copper, the copper, the surface of which has been covered with a photoresist, is also etched from the side thereof in etching the tin-nickel alloy film. This leads to variations in the length and width of the conductor line and a low accuracy in the resistance of the resistor circuit.
It is apparent from the fact that the tin-nickel alloy is not etched with an ammonia chelate based etching solution, the tin-nickel alloy is generally more stable against many chemicals than copper. For example, it is said that a tin-nickel alloy plated film of a thickness of more than 25.mu. has an anti-etching property between Monel metal and silver, almost comparable to Inconel.
In production of the printed circuit board with resistors wherein the tin-nickel alloy plated film is used as the resistor film, there has heretofore been employed a procedure wherein the laminated plate material is first dipped in an aqueous solution of hydrochloric acid and then in a mixed acid of sulfuric acid and nitric acid, in order to etch selectively the tin-nickel alloy plated film. With this etching solution, however, the selective etching of the tin-nickel alloy plated film in the presence of copper is not satisfactory.